feat: add display UI with splash screen, dashboard, and RSSI sparkline

Implement full SSD1306 OLED display support:
- Splash screen on boot (1.5s) with project name, version, board
- Dashboard showing radio state, config, counters, RSSI/SNR
- RSSI history sparkline (64-sample ring buffer, 2px-wide bars)
- DisplayOn/DisplayOff host commands routed via dedicated channel
- Pure render functions against generic DrawTarget (no hardware coupling)
- Display on/off control bypasses radio task entirely

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/cortex-m/Cargo.toml

@@ -0,0 +1,77 @@
+# THIS FILE IS AUTOMATICALLY GENERATED BY CARGO
+#
+# When uploading crates to the registry Cargo will automatically
+# "normalize" Cargo.toml files for maximal compatibility
+# with all versions of Cargo and also rewrite `path` dependencies
+# to registry (e.g., crates.io) dependencies.
+#
+# If you are reading this file be aware that the original Cargo.toml
+# will likely look very different (and much more reasonable).
+# See Cargo.toml.orig for the original contents.
+
+[package]
+edition = "2018"
+name = "cortex-m"
+version = "0.7.7"
+authors = [
+    "The Cortex-M Team <cortex-m@teams.rust-embedded.org>",
+    "Jorge Aparicio <jorge@japaric.io>",
+]
+links = "cortex-m"
+description = "Low level access to Cortex-M processors"
+documentation = "https://docs.rs/cortex-m"
+readme = "README.md"
+keywords = [
+    "arm",
+    "cortex-m",
+    "register",
+    "peripheral",
+]
+categories = [
+    "embedded",
+    "hardware-support",
+    "no-std",
+]
+license = "MIT OR Apache-2.0"
+repository = "https://github.com/rust-embedded/cortex-m"
+
+[package.metadata.docs.rs]
+targets = [
+    "thumbv8m.main-none-eabihf",
+    "thumbv6m-none-eabi",
+    "thumbv7em-none-eabi",
+    "thumbv7em-none-eabihf",
+    "thumbv7m-none-eabi",
+    "thumbv8m.base-none-eabi",
+    "thumbv8m.main-none-eabi",
+]
+
+[dependencies.bare-metal]
+version = "0.2.4"
+features = ["const-fn"]
+
+[dependencies.bitfield]
+version = "0.13.2"
+
+[dependencies.critical-section]
+version = "1.0.0"
+optional = true
+
+[dependencies.embedded-hal]
+version = "0.2.4"
+
+[dependencies.serde]
+version = "1"
+features = ["derive"]
+optional = true
+
+[dependencies.volatile-register]
+version = "0.2.0"
+
+[features]
+cm7 = []
+cm7-r0p1 = ["cm7"]
+critical-section-single-core = ["critical-section/restore-state-bool"]
+inline-asm = []
+linker-plugin-lto = []
+std = []